The present invention relates to flow meters in general, and in particular to flow meters that employ a wedge-shaped flow restricting element producing a pressure drop within the flow meter to indicate the volume of fluid flowing through the flow meter.
The use of wedges to create a pressure drop in flow meters for measurement of the volume of fluid passing through a flow meter is known in the art. Wedge-shaped flow meters are described in U.S. Pat. No. 4,237,739 issued on Dec. 9, 1980, and U.S. Pat. No. 4,926,698 issued on May 22, 1990. The ""739 patent describes a flow meter using a single wedge affixed to the internal wall of the flow meter whereas the ""698 patent describes a flow meter having two opposing wedges mounted on opposite sides of the flow meter interior wall. Either arrangement creates an opening within the flow meter having a reduced cross-sectional area in the flow-path of the fluid thereby creating a pressure differential on opposite sides of the wedge or wedges. The pressure differential created on opposite sides of the wedges has a known mathematical relationship to the flow rate of the fluid passing there through, and as long as the cross-sectional area of the opening at the wedge is constant, the fluid flow measurements are very accurate.
FIG. 1 shows a typical prior art flow meter 10 shown in cross-section. Flow meter 10 generally comprises a tubular housing 12 having a longitudinal passageway 14 in which a wedge-shaped member 16 is affixed to the inner wall 18 of housing 12 thereby creating at apex 20 of wedge 16 a restricted cross-sectional area represented by dimension D. At least two ports 22 are defined by housing 12. One of ports 22 is positioned upstream from wedge 16 and the other of ports 22 is positioned downstream from wedge 16. Ports 22 are in fluid communication with the interior flow through passage 14 thereby permitting the detection of the pressure differential induced by wedge 16 restricting fluid flow through flow meter 10.
Nevertheless, fluid flow conditions under which the flow meters are used are variable and tend to change. Specifically, temperature changes and changes in the pressure of the fluid being measured cause the diameter of the passageway through the flow meter to expand and contract. Consequently, the cross-sectional area between the wedge apex and the flow meter wall opposite the wedge does not remain constant. Small changes in the flow meter passageway diameter or the distance between the wedge apex and the wall opposite from the wedge can make substantial changes in the pressure drop of the fluid flowing past the wedge. Consequently, these changes introduce unwanted errors in the calculated volume of fluid flowing through the meter.
Thus, there is a need within the industry for a wedge-type flow meter where changes in the pressure and temperature of the fluid being measured by the flow meter will minimally affect the cross-sectional area of the restricted opening at the wedge. Such a flow meter will thereby provide improved accuracy in the measurement of the volume of fluid flowing through the meter.
One aspect of the present invention is a flow meter for measuring the volume of fluid flowing through the meter which includes an inner cylindrical tube through which the fluid flows and an outer cylindrical tube tending over the inner cylindrical tube. The outer cylindrical tube is radially spaced from the cylindrical inner tube to provide an annular cylindrical space between the inner cylindrical tube and the outer cylindrical tube. A seal between said inner cylindrical tube and the outer cylindrical tube closes the annular cylindrical space adjacent the ends of the annular cylindrical space. The inner cylindrical tube allows fluid pressure to enter the radial space between the inner cylindrical tube and the outer cylindrical tube through a surface opening opposite the sealed end of the inner cylindrical tube to provide pressure balancing between the pressure in the annular cylindrical space and the pressure in the inner cylindrical tube. A flow restriction member is mounted to an inner surface of said inner cylindrical tube for restricting the flow of fluid through the inner cylindrical tube and produces a pressure drop in the fluid as it flows past the flow restriction member. The inner cylindrical tube and the outer cylindrical tube in combination further define at least two ports for receiving a pressure sensing device to measure the pressure of the fluid flowing through said flow meter.
Another aspect of the present invention is a fluid flow meter for measuring the volume of fluid flowing through a passageway. The meter includes an outer housing having a first internal bore, and a removable inner member telescopically received in the first internal bore. The first internal bore and an outer surface of the inner member in combination define a cannular space therebetween wherein the cannular space is isolated from fluid flowing there through. The inner member has a second internal bore of a first predefined cross-sectional area to accommodate the fluid flow there through and is in pressure equalizing communication with the cannular space. A metering structure is mounted within the second internal bore for measuring the fluid flow there through.
Yet another aspect of the invention is a method for measuring the flow of a fluid through a tube. The method comprises the steps of providing an outer housing having an internal bore, and providing a calibrated tubular flow metering device having a flow restrictor mounted therein. The flow metering device is inserted within the outer housing internal bore in a telescoping fashion to create a cannular space between the outer housing and the metering device. The pressure of the cannular space is equalized with the internal pressure of the flow metering device. The combined outer housing and calibrated tubular flow metering device are coupled in the flow path of a fluid, and the pressure differential on each side of the flow restrictor is then measured.
These and other advantages of the invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims and appended drawings.